We investigate the stochastic migration of a Rydberg electron (RE) along its Coulomb energy spectrum in the variable electric field of a quasi-molecular ion produced in a symmetric collision between two alkali metal atoms. We give a kinetic description of the RE time evolution in the quasi-classical approximation using Fokker-Planck-type equations. The corresponding diffusion coefficient is derived in analytic form by taking into account the quantum defect for energy levels. We calculate the rate constants for diffusion ionization in Na**(nP) + Na(3S) collisions. Good agreement with experimental data is shown.